The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In general, large amounts of harmful substances to humans such as carbon monoxide and nitrogen oxides are contained in exhaust gases emitted from a vehicle engine. Strict regulations are being enforced on nitrogen oxides because the nitrogen oxides are particularly harmful in that they contribute to acid rain, global warming, and respiratory problems.
The nitrogen oxides have the property that, as the combustion temperature of fuel in the engine increases, so does the amount of nitrogen oxides.
Many attempts have been made to reduce nitrogen oxide emissions, among which an exhaust gas recirculation (EGR) system is usually applied to vehicles.
The EGR system recirculates part of the exhaust gas emitted from the engine after fuel combustion to an intake system of the engine to direct it back to a combustion chamber of the engine. As a consequence, an air-fuel mixture decreases in density without a change in the air-fuel ratio of the air-fuel mixture, thus lowering the combustion temperature.
That is, the EGR system supplies a portion of exhaust gas to an intake manifold of the engine to direct it to the combustion chamber when there is a need to reduce nitrogen oxide emissions depending on the operating state of the engine. By doing so, exhaust gases help to decrease the density of the mixture to a lower level and therefore decrease the flame propagation velocity during fuel combustion. This suppresses an increase in combustion temperature and slows the fuel combustion, thereby suppressing the generation of nitrogen oxides.
Some EGR systems use a dedicated EGR cylinder type engine, where one engine cylinder supplies a high volume of exhaust gas to the EGR system at rates approaching 25% of exhaust gas being recirculated to the intake system of the engine. Such dedicated EGR systems can provide a slight supercharging effect. To attempt to control exhaust gas delivery to engine inlets equally and thereby increase engine stability, existing dedicated cylinder EGR systems attempt to restrict the exhaust gas recirculation flow and slow the passage of the EGR into the engine inlet stream. We have discovered that such existing dedicated cylinder EGR systems can be difficult to design and implement correctly, result in high pumping work and loss of fuel economy, and tend to be only effective at certain engine operating conditions due to varying gas flow rates.